The present invention relates to an image transfer system comprising an endless belt serving as an image carrier and passing through a first processing station where it is kept at a low temperature and through a second processing station where it is kept at a higher temperature, and a counter flow heat exchanger formed by two portions of said belt moving in opposite directions and held in sliding contact with each other by a pressing member.
An image transfer system is used for example in a copier or printer in which a toner image is developed on or transferred onto an image carrier in the first processing station and is then transferred onto a sheet of copy paper and fused thereon in the second processing station. Especially when the transfer step and the fuse step in the second processing station are combined into a single transfusion step, the belt needs to be heated to an elevated temperature before it is fed to the second processing station. On the other hand, the first transfer step or the developing step in the first processing station generally requires a lower belt temperature. In a specific type of copying or printing machine, the toner image is developed on a photoconductive drum or is directly formed on the surface of an electrode drum in a direct induction printing process and is then transferred to the belt which serves as an intermediate image carrier.
The arrangement in which portions of the belt are configured as a counter flow heat exchanger has the advantage that the losses of heat energy and hence the power consumption of the machine can be reduced significantly because a substantial part of the heat of the belt leaving the second processing station can be recovered and used for pre-heating the belt which is fed to the second processing station. In order to achieve a high efficiency of the heat exchanger and to reduce the length of the heat exchanger, it is preferable that the belt is formed by a relatively thin endless support and is made of a material which has a small heat capacity and a high thermal conductivity. Further, the two portions of the belt forming the heat exchanger should be held in close contact with each other. On the other hand, the force with which these two belt portions are pressed against one another should not be too large in order to limit the amount of friction between the belt surfaces.
U.S. Pat. No. 5,103,263 discloses an image transfer system of the type indicated above in which the heat exchanger is configured as a straight path defined between two deflecting rollers. One of the belt portions moves tangentially past the two deflecting rollers without being substantially deflected, whereas the other portion is deflected at both rollers so as to be in sliding contact with the first portion only in the straight path between the two deflecting rollers. A plate-like pressing member is used for slightly pressing the second belt portion against the first one over the length of the heat exchanger.
It is an object of the present invention to improve the efficiency of the heat exchanger and to thereby allow for a more compact construction of the overall system.
According to the present invention, this object is achieved by the feature that the pressing member has a curved surface over which portions of the belt are guided over a substantial part of the length of the counter flow heat exchanger, and the curved surface co-rotates with the belt portion that is in direct contact therewith.
Thus, the heat exchanger is essentially formed by a curved path on which the two belt portions are superposed on the curved surface of the pressing member. As a result, the outer portion of the belt is pressed against the inner one, which is directly supported by the pressing member, with a force that is proportional to the belt tension and increases with increasing curvature of the pressing member. As a result, the two belt portions moving in opposite directions are held in close contact with each other due to the pressing force which can be finely adjusted by appropriately selecting the belt tension. This assures a good reproducible heat transfer from one belt layer to the other, so that the efficiency of the heat exchanger is increased and the length thereof can be reduced. As will be illustrated hereinafter, the efficiency of the heat exchanger is remarkably increased in comparison with the configuration as shown in U.S. Pat. No. 5,103,263.
In addition, the curved configuration of the counter current heat exchanger makes it possible to arrange the first and second processing stations in relatively close proximity to one another, in spite of the necessary length of the heat exchanger. As a result, a compact construction of the overall system is achieved.
It will be further understood that the second processing station operating at higher temperature should be insulated against thermal losses, so that, ideally, the heat is dissipated only through the heat exchanger.
According to the present invention, the pressing member is a cylindrically shaped member which co-rotates with one of the belt portions forming the heat exchanger. Thus, no friction will occur between the pressing member and the belt portion directly supported thereon. Friction will occur only between the opposed surfaces of the two belt layers superposed on the circumference of the rotating drum. These surfaces, however, which are not the image carrying surfaces of the belt, may have a comparatively low friction coefficient, so that the frictional resistance is minimised.
The drum forming the pressing member should be made of a material which has a small heat conductivity and a small heat capacity, so that the heat of the hot portion of the belt will be dissipated substantially to the cooler belt portion but not to the drum serving as a pressing member.
In order to further minimise the thermal contact between the belt and the drum, it is preferable to provide a pattern of grooves on the surface of the drum, so that the drum will be in contact with the belt only in ridge or island portions defined between the grooves. Of course, the width of the grooves should be small enough to avoid any distortion of the belt layers which could have a negative effect on the image quality or on the durability of the belt and to assure appropriate thermal contact of the entire belt areas in the heat exchanger.
In order to increase the temperature of the belt up to the process temperature needed in the second processing station, a heater will generally be arranged between the heat exchanger and the second processing station. As a result, there exists a temperature difference between the belt portion exiting from the heat exchanger toward the second processing station and the portion re-entering from the second processing station into the heat exchanger. Such a temperature difference is necessary for maintaining the heat transfer from one belt layer to the other within the heat exchanger. If thermal losses are reduced to such an extent that they may be neglected, conservation of energy requires that the same temperature difference is present between the two belt layers over the whole length of the heat exchanger. Consequently, this temperature difference will also be present between the belt portion exiting from the heat exchanger towards the first processing station and the portion re-entering into the heat exchanger from the first processing station. This means that the heat energy which has been supplied by the heater on the side of the second processing station must be extracted on the side of the first processing station.
In order to achieve a compact construction, this heat extraction is preferably promoted by an active cooling system, e.g. an air or a water-cooled drum which preferably deflects the belt by a comparatively large angle, e.g. an angle of about 180xc2x0. For example, the cooling drum may be situated upstream of the first processing station and may be arranged to cool the belt to a temperature sufficient low to avoid that the first procession station is warmed up by the belt above the operating temperature of the first processing station. This design has the advantage that the cooling function is completely removed from the first processing station itself, so that the component parts forming the first processing station can be reduced in complexity.